Method and device for crucible-free zone melting a crystalline rod

ABSTRACT

Method of crucible-free zone melting a crystalline rod, such as a semiconductor rod especially, wherein the rod is surrounded by an induction heating coil energizable so as to form a molten zone in the rod, includes passing the molten zone axially through the rod, and simultaneously maintaining the crystalline rod and the induction heating coil at the same electric potential during passage of the molten zone through the rod; and device for carrying out the method.

United States Patent Keller [54] METHOD AND DEVICE FOR CRUCIBLE-FREE ZONE MELTING A CRYSTALLINE ROD [72] Inventor: Wollgang Keller, Pretzfeld, Germany [73] Assignee: Slemens Aktlengesellsehaft, Berlin and Mynishi Ge [221 Filed: Mar.13,1970

211 Appl.No.: 19,290

[30] Foreign Application Priority Data Mar. 19,1969 Germany ..P 19 13 881.9

[52] US. Cl ..148/ 1.6,23/273 SP, 23/301 SP [51] Int. Cl. B0lj 17/10 [58] Field of Search ..l48/l .6; 23/273 SP, 301 SP;

[ 5] Feb. 22, 1972 [56] References Cited UNITED STATES PATENTS 3,058,915 10/1962 Bennett ..23/273 X 3,157,472 11/1964 Kappelmeyer et al. ..23/273 X Primary Examiner-L. Dewayne Rutledge Assistant Examiner-E. L. Weise Attorney-Curt M. Avery, Arthur E. Wilfond, Herbert L. Lerner and Daniel .1. Tick ABSTRACT Method of crucible-free zone melting a crystalline rod. such as a semiconductor rod especially, wherein the rod is surrounded by an induction heating coil energizable so as to form a molten zone in the rod, includes passing the molten zone axially through the rod, and simultaneously maintaining the crystalline rod and the induction heating coil at the same electric potential during passage of the molten zone through the rod; and device for carrying out the method.

10 Claims, 6 Drawing Figures PATENTEDFEB22 m2 3.644.151

SHEET 2 [IF 2 'METI-IOD AND DEVICE FOR CRUCIBLE-FREE ZONE MELTING A CRYSTALLINE ROD My invention relates to method and device for cruciblefree, floating zone melting of a crystalline rod.

Devices for crucible-free zone melting a crystalline rod frequently include, in a melting chamber in addition to rod end holders, an induction heating coil for heating the molten zone. By means of relative displacement of the rod holders, on the one hand, and the induction heating coil, on the other hand, the molten zone is passed through the crystalline rod. The induction heating coil can be, for example, a multiwinding cylinder coil and is also often in the form of a coil having a single winding. The induction heating coil is supplied or energized with high-frequency alternating current from a highfrequency generator.

The melting chamber of the conventional zone melting devices is largely evacuated, but can also be filled with a protective gas, such as highly pure hydrogen, wherein rod-shaped monocrystals, having especially good crystal quality, can be produced by crucible-free zone melting.

' Particularly, single-winding coils and cylinder coils tend toward electrical flashovers in the course of a crucible-free zone melting process. These flashovers can have damaging effe'cts on the crystal quality of the crystalline rod-produced by the crucible-free zone melting process. Danger of flashovers is especially great in a melting chamber filled with protective gas and/or when the heating coil is operated at high electrical power.

It is accordingly an object of my invention to provide method and device for crucible-free zone melting a crystalline rod which avoids the foregoing disadvantages of the heretofore known methods and devices of this general type, and which, more particularly, avoids flashovers.

With the foregoing and other objects in view, I provide, according to my invention, in a method of crucible-free zone melting a crystalline rod, such as a semiconductor rod especially, wherein the rod is surrounded by an induction heating coil energizable so as to form a molten zone in the rod, passing the molten zone axially through the rod and applying and maintaining the same electric potential both to the crystalline rod and the induction heating coil during passage of the molten zone through the rod.

In accordance with the device for carrying out the foregoing method of the invention, I provide a melting chamber, holders mounted in the melting chamber for end supporting a rod, an induction heating coil mounted in the melting chamber for heating a melting zone formed in the rod, means for electrically connecting a middle portion of the induction heating coil to at least one of the rod holders.

In accordance with a further feature of my invention, I provide an electrical balancing or equalizing unit connected in parallel to the induction heating coil and having a center tap electrically connected to at least one of the rod holders.

Other features which are considered as characteristic for the invention are set forth in the appended claims.

Although the invention is illustrated and described herein as method and device for crucible-free zone melting a crystalline rod, it is nevertheless not intended to be limited to the details shown, since various modifications may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. The invention, however, together with additional objects and advantages thereof will be best understood from the following description when read in connection with the accompanying drawing, in which:

FIG. I is a schematic view partly in section of a device according to my invention, for crucible-free zone; melting a crystalline rod having an induction heating coil and a highfrequency generator for energizing the heating coil;

FIG. 2a is a view similar to that of FIG. 1 of another embodiment thereof having an electrical balancing unit or component connected in parallel with the induction heating coil;

FIGS. 2!: to 2d are diagrammatic views of different embodiments of the electrical balancing component of FIG. 2a.

FIG. 3 is another view similar to that of FIG. 2a of the device showing another embodiment of electrical balancing circuit. Referring now to the drawing and particularly to FIG. 1 thereof, there is shown a section of the metal wall 101 of a melting chamber for a device for crucible-free floating-zone melting of a crystalline rod. Fittings 102 pass through suitable openings formed in the wall 101, and metal shafts 103 extend through. the fittings 102 into the melting chamber and carry metal rod holders 104 at the ends thereof. The fittings 102 are gastightly secured with an oil seal 105. The shafts 103 and, accordingly, the rod holders 104 secured to the ends thereof, are rotatable about the longitudinal axes thereof and are also displaceable in axial direction thereof, if desired. An end of a crystalline rod 106, for example a rod of germanium or silicon, is secured respectively in each of the rod holders 104. With the aid of an induction heating coil 107, which, as illustrated, is a coil having a single winding but which can, however, for example, also be a cylindrical coil having a plurality of windings, a molten zone 108 is produced in the rod 106 and is passed through the rod 106 by relative motion between the rod 106 and the induction heating coil 107. The melting chamber, which is not illustrated in its entirety in FIG. I, is filled, for example, with highly pure hydrogen as protective gas but can also, if desired, be subjected to high vacuum.

Capacitors 115 are connected in parallel to the induction heating coil 107 and form therewith a heating circuit, i.e., a resonant or oscillating circuit, which is supplied with electrical energy from a high-frequency generator 110 through a coaxial cable 109. The coaxial cable 109 and the heating circuit formed of the induction heating coil 107 and the capacitors 108 are coupled through a coupler or coupling coil 112 to the moving coil 1110f a tank circuit in the high-frequency generator 110.

The middle winding portion 113 of the single winding induction heating coil 107 or the middle winding of a multiwinding cylindrical induction heating coil (not shown) is connected to the wall 101 of the induction chamber through an electrical conductor 114. Both the rod holders 104 and consequently the rod. 106 as well as the middle winding portion 113 and the induction heating coil 107 are at the same potential because the shaft 103 and the wall 101 of the melting chamber are grounded. Thereby, the maximum difference in potential between the induction heating coil 107 and the crystalline rod 106, is only half as large as it would have been if the middle winding portion 113 of the induction heating coil 107 were not electrically conductively connected to the melting chamber wall 113 and would therefore not have had the same potential as the rod 106. Consequently, no flashovers occur in the device shown in FIG. I. The circuit of the induction heating coil of FIG. 1 is especially advantageous if the induction heating coil is stationarily mounted in the melting chamber wall and the rod which is to be subjected to the zone melting process is displaced in the axial direction thereof relative to the coil 107 and the melting chamber. The section of a device for crucible zone melting shown in FIG. 2a corresponds to that of FIG. 1. Similar elements in both of the figures as well as in the remaining figures of the drawings, are identified by the same reference numerals. The middle winding portion 113 of the induction heating coil 107 is not, however, maintained as in FIG. 1, through an electrical conductor connected to the melting chamber coil 101 at the potential of the holders 104 and consequently, of the semiconductor rod 106, but rather through an electrical equalizing or balancing unit or component having a center tap which is connected through an electrical conductor 116 to the melting chamber wall 101. Since the melting chamber wall 101 and the holders 104 are grounded, the center tap and the crystalline rod 106 are at the same potential. The balancing unit shown in FIG. 2a is formed of three groups ll7 to 119, respectively, consisting of two capacitors connected in series. These groups 117 to 119 are connected in parallel to the induction heating coil 107. The capacitance of the capacitors in each group is substantially of equal value. The middle tap of this capacitance divider circuit is formed of an electrical conductor 120 through which the individual capacitors in each group 117 to 119 are electrically connected by an electrical conductor 116 to the melting chamber wall 101. A capacitance divider circuit as a balancing unit such as is shown in FIG. 2a has the advantage that the capacitor groups 117 to 119 serve simultaneously as resonant circuit capacitors for the heating circuit with the induction heating coil 107.

FIG. 2b shows a further capacitive divider circuit which can serve as electrical balancing unit in a device according to FIG. 2a.'ln addition to two heating circuit capacitors 121 and 122 connected in parallel to the induction heating coil 107, there is provided a group 123 formed of two series-connected capacitors 124 and 125 of the same capacity as the electrical balancing unit which is connected in parallel to the induction heating coil 107. The center tap 126 of this balancing unit is the connecting conductor between the capacitor 124 and 125. This center tap is ,electrically connected to the coil of the grounded melting chamber by an electrical conductor 116.

FIG. 2c shows an inductive divider circuit as balancing unit. A coil 128, having a greater inductivity in comparison to the inductivity of the induction heating coil 107, is connected in parallel to a heating circuit formed of the induction heating coil 107 and the capacitors 127. Preferably, the inductivity of the coil 128 is greater by about a factor of than that of the induction heating coil 107. The power loss in the coil 128 is thereby kept very low. The coil 128 has a center tap 129 which is electrically connected through an electrical conductor 1 16 with the grounded melting chamber wall.

An additional inductive divider circuit is shown in FIG. 2d wherein the heating circuit is formed of the induction heating coil 107 and capacitors 131 connected in parallel with the induction heating coil 107. The middle winding or the middle winding portion of the coupling coil serving as the balancing unit is electrically connected by means of an electrical conductor 1 16 with the grounded wall of the melting chamber.

The circuits according to FIGS. 2a to 20 and particularly the inductive divider circuits according to FIG. 2d are especially suitable for zone melting devices with displaceable coils because they do not require any electrical supply lines to the induction heating coil to be connected to the melting chamber wall. For reasons of construction it can be advantageous to insulate the shaft 103 and the rod holders 104 as well as the zone melting rod 106 with respect to the wall of the melting chamber and, accordingly, with respect to ground. In this case, for example, a branch conductor of the coaxial cable leading from the induction coil to the heating circuit can be grounded. FIG. 3 illustrates a section of a device for cruciblefree zone melting wherein such a circuit is shown. The shafts 103 are provided with insulating portions 132 which insulate the holder 104 from the melting chamber wall 101. In FIG. 3 there is shown a capacitive divider circuit according to FIG. 2b as the balancing unit, however, a device circuit corresponding to those in FIGS. 2a, 2c and 2d can be substituted therefor. The center tap 126 of the electrical balancing unit or component in the device according to FIG. 3 is electrically connected by means of an electrical conductor 116 to a contact tongue or blade 133. The contact blade 133 produces an electrical contact with the shaft 103 between the lower winding portion 132 and the lower holder 104, as viewed in FIG. 3, and accordingly with the crystalline rod 106 that is being zone-melted.

Instead of employing an electrical equalizing or balancing unit, the middle winding portion 113 of the induction heating coil 107 in the device according to FIG. 3, can be maintained at the same potential as that of the rod 106 by connecting the middle winding portion 113 to the contact blade 133 by means of electrical conductors.

I claim:

1. In a method of crucible-free zone melting a crystalline rod, such as a semiconductor rod especially wherein the rod is surrounded by an induction heating coil energizable so as to form a molten zone in the rod, passingthemolten zone axially through the rod, and applying and maintaining the same electric potential both to the crystalline rod and the induction heating coil during passage of the molten zone through the rod.

2. Device for crucible free zone melting a crystalline rod, such as a semiconductor rod especially, comprising a melting chamber, holders mounted in said chamber for end supporting a crystalline rod to be zone melted, an induction heating coil mounted in said chamber for heating a melting zone in the rod, and means for electrically connecting a middle portion of the induction heating coil to at least one of said rod holders.

3. Device according to claim 2 wherein at least one of said rod end holders, said middle portion of said induction heating coil and said melting chamber are electrically connected to ground.

4. Device according to claim 2 wherein said rod end holders and said middle portion of said induction heating coil are electrically insulated fromsaid melting chamber.

5. Device for crucible-free zone melting a crystalline rod, such as a semiconductor rod especially, comprising a melting chamber, holders mounted in said chamber for end supporting a crystalline rod to be zone melted, an induction heating coil mounted in said chamber for heating a melting zone in the rod, an electrical balancing unit connected in parallel with said induction heating coil, said electrical balancing unit having a central tap electrically connected to at least one of said rod end holders.

6. Device according to claim 5 wherein said balancing unit is a capacitive divider circuit.

7. Device according to claim 5 wherein said balancing unit is an inductive divider circuit.

8. Device according to claim 7 including a high-frequency generator having a tank circuit, and said induction heating coil is connected in a heating circuit with a coupling coil, said heating circuit being inductively coupled by said coupling coil with said tank circuit of said high-frequency generator, said coupling coil having a middle winding portion electrically connected to at least one of said rod end holders.

9. Device according to claim 5 wherein said center tap, at least one of said rod end holders and said melting chamber are connected to ground.

10. Device according to claim 5 wherein said center tap and said rod end holders are electrically insulated from said melting chamber. 

2. Device for crucible free zone melting a crystalline rod, such as a semiconductor rod especially, comprising a melting chamber, holders mounted in said chamber for end supporting a crystalline rod to be zone melted, an induction heating coil mounted in said chamber for heating a melting zone in the rod, and means for electrically connecting a middle portion of the induction heating coil to at least one of said rod holders.
 3. Device according to claim 2 wherein at least one of said rod end holders, said middle portion of said induction heating coil and said melting chamber are electrically connected to ground.
 4. Device according to claim 2 wherein said rod end holders and said middle portion of said induction heating coil are electrically insulated from said melting chamber.
 5. Device for crucible-free zone melting a crystalline rod, such as a semiconductor rod especially, comprising a melting chamber, holders mounted in said chamber for end supporting a crystalline rod to be zone melted, an induction heating coil mounted in said chamber for heating a melting zone in the rod, an electrical balancing unit connected in parallel with said induction heating coil, said electrical balancing unit having a central tap electrically connected to at least one of said rod end holders.
 6. Device according to claim 5 wherein said balancing unit is a capacitive divider circuit.
 7. Device according to claim 5 wherein said balancing unit is an inductive divider circuit.
 8. Device according to claim 7 including a high-frequency generator having a tank circuit, and said induction heating coil is connected in a heating circuit with a coupling coil, said heating circuit being inductively coupled by said coupling coil with said tank circuit of said high-frequency Generator, said coupling coil having a middle winding portion electrically connected to at least one of said rod end holders.
 9. Device according to claim 5 wherein said center tap, at least one of said rod end holders and said melting chamber are connected to ground.
 10. Device according to claim 5 wherein said center tap and said rod end holders are electrically insulated from said melting chamber. 